Semiconductor gas detecting device

ABSTRACT

A gas detecting device formed of a porous, highly permeable, metal oxide semiconductor formed by mixing semiconductor particles with inorganic materials which will render the semiconductor highly porous or by intermixing the semiconductor with materials which can be evaporated or otherwise removed by heating to a temperature below the sintering temperature of the semiconductor. The resultant semiconductor material is contained within a suitable housing that will permit gases to flow readily through the semiconductor and appropriate electrodes are affixed to the semiconductor to measure a change in resistance in the presence of reducing gases. Heating means may be utilized to stabilize the resistance of the semiconductor material during operation and such heating means may comprise at least one of the electrodes.

United States Patent [1 1 Taguchi 1 May 8, 1973 [54] SEMICONDUCTOR GASDETECTING DEVICE [76] Inventor: Naoyoshi Taguchi, 6-8, 2-chome Hyuga,Tarumi-ku, Kobe, Japan [22] Filed: Sept. 23, 1970 [21] Appl. No.: 74,749

UNITED STATES PATENTS 3,625,756 12/1971 Taguchi ..338/34 3,603,9549/1971 3,027,552 3/l962 3,144,850 8/1964 Primary Examiner-R. SkudyAtmrney-Eugene E. Geoffrey, Jr.

[57] ABSTRACT A gas detecting device formed of a porous, highlypermeable, metal oxide semiconductor formed by mixing semiconductorparticles with inorganic materials which will render the semiconductorhighly porous or by inter-mixing the semiconductor with materials whichcan be evaporated or otherwise removed by heating to a temperature belowthe sintering temperature of the semiconductor. The resultantsemiconductor material is contained within a suitable housing that willpermit gases to flow readily through the semiconductor and appropriateelectrodes are affixed to the semiconductor to measure a change inresistance in the presence of reducing gases. Heating means may beutilized to stabilize the resistance of the semiconductor materialduring operation and such heating means may comprise at least one of theelectrodes.

7 Claims, 10 Drawing Figures PATENTED 81975 3,732,519

SHEET 2 OF 2 INVENTOR. /V40 YOSH/ ll zjcm.

1 SEMICONDUCTOR GAS DETECTING DEVICE This invention relates to a gasdetecting device and, more specifically, to a gas detector embodying ametal oxide semiconductor and method of manufacturing having aresistivity which undergoes a substantial change when the semiconductorcontacts a reducing gas, such as hydrogen, propane, butane or smoke.

Prior gas detecting devices utilized metal oxide semiconductors foradsorbing gases wherein the semiconductor was sintered after beingformed in a predetermined configuration. Preshaped metal oxidesemiconductors were sintered in order to improve the mechanical strengthbut in so doing the material enters a semi-molten state which reducesits air permeability with the result that its gas adsorptioncharacteristics are substantially reduced and a severe reduction ofsensitivity occurs.

One object of the invention resides in the provision of a novel improvedgas detecting device embodying a metal oxide semiconductor ofsubstantial mechanical strength and having a high degree of airpermeability which permits gases to readily permeate the structure andthereby affords a relatively high degree of sensitivity.

Another object of the invention resides in the provision of a novel andimproved method for manufacturing gas detecting devices embodying metaloxide semiconductors of substantial mechanical strength and having ahigh degree of air permeability.

The device in accordance with the invention includes a porous metaloxide semiconductor block having a high degree of permeability andexhibiting a substantial change in electroconductivity in the presenceof reducing gases and at least two electrodes electrically contactingtwo separate portions of the block in order to detect changes inelectroconductivity.

In the manufacture of the device in accordance with the invention, apowdered metal oxide semiconductor material having anelectroconductivity which will undergo substantial change in thepresence of reducing gases is shaped into the form of a block at a lowtemperature which neither fuses nor sinters the powder and at least onepair of electrodes which are secured to separate portions of the blockin order to measure changes in electroconductivity.

The above and other objects and advantages of the invention will becomemore apparent from the following description and the accompanyingdrawings forming part of this application.

IN THE DRAWINGS FIG. 1 is a vertical cross-sectional view of oneembodiment of a gas detecting device in accordance with the invention;

FIG. 2 is a side elevational view of a modified embodiment of a gasdetecting device in accordance with the invention;

FIG. 3 is a cross-sectional view of FIG. the line 3-3 thereof;

FIG. 4 is a side elevational view of still another embodiment of a gasdetecting device in accordance with the invention;

FIG. 5 is a cross-sectional view of FIG. 4 the line 5-5 thereof;

FIG. 6 is a side elevational view of a still further embodiment of theinvention;

2 taken along taken along FIG. 7 is a cross-sectional view of FIG. 6taken along the line 7-7 thereof;

FIG. 8 is a side elevational view of still another embodiment of theinvention;

FIG. is a cross-sectional view of FIG. 8 taken along the line 9-9thereof; and

FIG. 10 is a circuit diagram of one embodiment in an alarm systemembodying a gas detecting device in accordance with the invention.

The embodiment of the invention illustrated in FIG. 1 includes acylindrical metal casing 2 serving as one electrode and enclosing apreshaped block of a metal oxide semiconductor material 4 having quartzparticles 6 dispersed therein. The metal oxide semiconductor material 4may be formed of one of the reduction type semiconductors, such as SnOZnO, Fe O or TiO These materials increase their electroconductivities inresponse to adsorption of reducing gases. If desired, an oxidation typesemiconductor, such as NO, C00 or Cr O may be utilized in which eventthe conductivity will decrease in the presence of a reducing gas.

Particles 6 dispersed throughout the semiconductor may be formed ofalumina or other inorganic substance in place of quartz. The particles 6are preferably coarser than the metal oxide semiconductor powder 4 andshould have relatively rough, uneven surfaces. The ends of the casing 2are closed by covers 8 and 10 formed of a permeable material to permitgases to flow therethrough. A heater 12 formed of resistive wire woundabout an insulator is embedded in the semiconductor and conductors 14connected to the ends of the heater extend outwardly through the cover10. The heater 12 also serves as a second electrode.

When the metal oxide semiconductor 4 is of a reduction type, the gasdetecting device would be connected as illustrated in FIG. 10. Morespecifically, the terminals 20 and 22 of the secondary winding ontransformer 18 are connected to the conductors 14 of the heater 12. Athird terminal 24 on the secondary winding is connected through a buzzeror other alarm 26 to a conductor 16 electrically connected to the casing2. The primary winding of the transformer 18 is connected to a suitablepower supply. The transformer 18 is designed in such a manner that asufficient voltage will be produced between the terminals 20 and 22 tocause a current of sufficient magnitude to flow through the heater 12 toheat the semiconductor material to a temperature of the order of C. to200 C. Heating of the semiconductor stabilizes its resistance duringoperation and also increases the change in resistance when the detectoris placed in an atmosphere containing a predetermined concentration of agas to be detected.

When the gas detecting device 1 is not exposed to a reducing gas, theresistance of the metal oxide semiconductor material 4 will berelatively large with the result that a current will flow through thebuzzer 26 which is too small to activate the buzzer. When the detectingdevice I is exposed to a reducing gas, the resistance of thesemiconductor 4 decreases and this in turn substantially increases thecurrent flowing through the buzzer and energizes the buzzer to sound analarm.

When the metal oxide semiconductor 4 is of the 0xidation type, itincreases its resistance when exposed to a reducing gas. Under theseconditions, a relay would be utilized having contacts which close whenthe current through the relay is small. The coil of the relay would beconnected between the terminal 24 of the secondary winding on thetransformer 18 and the conductor 16 of the gas detecting device 1. Thebuzzer 26 would then be connected to a suitable power supply through thecontacts of the relay. It is, of course, understood that the metal oxidesemiconductor material, whether of the reduction type or oxidation type,returns to its normal resistance when the contaminating gas is removedfrom the ambient atmosphere.

Inasmuch as the particles 6 in the gas detecting device in accordancewith the invention have relatively rough and uneven surfaces, theeffective area of the semiconductor material 4 which can adsorb gases isgreatly increased with the result that the sensitivity of the device issubstantially increased.

If desired, the particles 6 may be replaced by holes or voids to providefurther improvement in sensitivity. To effect this end, particles ofstarch, such as flour, crushed rice, or dog-tooth violet starch, aremixed with a semiconductor powder and the mixture is shaped underpressure and then heated to a sufficient temperature to burn away orevaporate the starch and at the same time prevent sintering of thematerial. This procedure causes pores to be formed in the semiconductorblock so that the effective gas absorbing area is substantiallyincreased with an attendant increase in its gas detection sensitivity.

FIGS. 2 and 3 show the modified embodiment of the invention and utilizea central body 30 of ceramic or other suitable material havingelectrodes 32 and 34 on opposite sides thereof. A porous metal oxidesemiconductor film 36 is formed about the entire exposed surface of thebody 30 as well as at least part of the electrodes 32 and 34. Conductors42 and 44 are connected to the electrodes 32 and 34, respectively.

The manufacture of the porous metal oxide semiconductor material layer36 will be described utilizing a typical material, such as SnO as thesemiconductor. One gram of SnCl is added to 8 grams stearic acid as abinder. Though the weight ratio is not critical, fumes produces by SnCL,when too small an amount of stearic acid is utilized will adverselyaffect the health of the operators. The mixture is then heated toproduce a dark brown liquid and the liquid is then applied to the entiresurface of the body 30 and part of the electrodes 32 and 34. Thestructure is then baked in air to produce the metal oxide semiconductorlayer 36 consisting of SnO- as illustrated in the drawings. If desired,the liquid may be sprayed on a preheated base 30. The resultant metaloxide semiconductor layer 36 consisting of Sn will have a large numberof pores formed by the stearic acid which has a high molecular weightwhen SnO crystals are produced. In addition to the stearic acid, otherbinders may be utilized which have high molecular weights and can beburned away as, for instance, wax, sugar, polyvinyl alcohol or acrylicresins.

The foregoing embodiment of the invention has a very large change inresistance between electrodes 32 and 34 when subjected to a gascontaminated atmosphere with the result that its sensitivity is high.This substantial increase in sensitivity results from the fact that thelayer 36 is extremely porous and has a very large absorbing area.Furthermore, the metal oxide layer 36 has sufficient mechanical strengthsince it is carried by the base 30. Although a heater for heating thesemiconductor 36 has not been illustrated in this embodiment of theinvention, a suitable heater may be employed to. heat the layer 36, ifdesired.

FIGS. 4 and 5 illustrate another embodiment of a gas detecting device inaccordance with the invention. In this embodiment, a metal casing 50surrounds a metal oxide semiconductor block 56 which is merely formed bypressure shaping a powdered metal oxide semiconductor material in whichelectrodes 52 and 54 have been embedded.

Since the semiconductor block 56 of this embodiment is merely formed bypress-shaping the powdered material, the material is extremely porousand exhibits a very large change in resistance between the electrodes 52and 54 in the presence of a contaminated atmosphere. Furthermore, thesurrounding metal casing 50 provides a high degree of mechanicalstrength. If desired, a heater may be utilized for heating thesemiconductor block 56.

A further. embodiment of a gas detecting device in accordance with theinvention is illustrated in FIGS. 6 and 7. This form of the inventiondiffers from the form of the invention shown in FIGS. 4 and 5 in that asingle electrode 62 is embedded in the semiconductor material 64 and thecasing surrounding the semiconductor 64 is utilized as the secondelectrode. The semiconductor material 64 may be formed in the samemanner as described in connection with the structure shown in FIGS. 4and 5.

A further embodiment of the invention is shown in FIGS. 8 and 9. In thisembodiment, the metal oxide semiconductor block 74 is formed by apress-shaping process and is forced into a cylinder which is formed bybonding two semicylindrical metal electrodes 66 and 68 to insulators 70and 72. The semiconductor material 74 may be formed in the same manneras previously described in connection with FIGS. 4 and 5 and affords ahigh degree of sensitivity to gas contaminated ambient air.

The several embodiments described above are for illustrative purposesonly and various modifications and changes may be made without departingfrom the spirit and scope of the invention. For example, the electrode62 of the embodiment shown in FIGS. 6 and 7 may be replaced by a singleheater wire extending through the semiconductor material and alsofunctioning as the electrode 62. The cylindrical housing 60 may be ofcup shape. In all of the modifications, an alarm device is operated byutilizing the change of resistance of the metal oxide semiconductormaterial when it contacts a gas contaminated atmosphere.

. The semiconductor materials 56, 64 and 74- may be formed in the samemanner as the semiconductor material 4.

What is claimed is:

l. A gas detecting device comprising a porous and permeable metal oxidesemiconductor body including semiconductor particles which exhibits achange in electroconductivity when it contacts a reducing gas in theair, and at least one pair of electrodes attached to said body and inelectrical contact therewith, said body including particles of materialselected from the group consisting of alumina and quartz.

2. A gas detecting device comprising a porous metal oxide semiconductorbody including semiconductor particles which exhibits a change inelectrical conductivity when it contacts a reducing gas in air, at leastone pair of electrodes attached to said body and in electrical contacttherewith, said metal oxide semiconductor body being contained within acasing with at least a portion of said body being exposed to air.

3. A gas detecting device, in accordance with claim 2 wherein saidcasing is made of metal and serves as one of said electrodes and theother of said electrodes is embedded in said body.

4. A gas detecting device comprising a porous metal oxide semiconductorbody including semiconductor particles which exhibits a change inelectrical conductivity when it contacts a reducing gas in air, at leastone pair of electrodes attached to said body and in electrical contacttherewith, said metal oxide semiconductor body being contained within acasing with at least a portion of the surface of said body being exposedto the air, said pair of electrodes being inserted in said metal oxidesemiconductor body contained in said casing.

5. A gas detecting device, in accordance with claim 2 wherein saidcasing comprises two portions formed of metal with said portions beingelectrically insulated from each other and serve as electrodes.

6. A gas detecting device comprising a porous metal oxide semiconductorbody including semiconductor particles which exhibits a change inelectrical conductivity when it contacts a reducing gas in air, at leastone pair of electrodes attached to said body and in electrical contacttherewith, said metal oxide semiconductor body being in the form of alayer on the surface of an insulator.

7. A gas detecting device comprising a porous metal oxide semiconductorbody including semiconductor particles which exhibits a change inelectrical conductivity when it contacts a reducing gas in air, at leastone pair of electrodes attached to said body and in electrical contacttherewith, and heating means for said metal oxide semiconductor body tomaintain it at a predetermined temperature.

2. A gas detecting device comprising a porous metal oxide semiconductorbody including semiconductor particles which exhibits a change inelectrical conductivity when it contacts a reducing gas in air, at leastone pair of electrodes attached to said body and in electrical contacttherewith, said metal oxide semiconductor body being contained within acasing with at least a portion of said body being exposed to air.
 3. Agas detecting device, in accordance with claim 2 wherein said casing ismade of metal and serves as one of said electrodes and the other of saidelectrodes is embedded in said body.
 4. A gas detecting devicecomprising a porous metal oxide semiconductor body includingsemiconductor particles which exhibits a change in electricalconductivity when it contacts a reducing gas in air, at least one pairof electrodes attached to said body and in electrical contact therewith,said metal oxide semiconductor body being contained within a casing withat least a portion of the surface of said body being exposed to the air,said pair of electrodes being inserted in said metal oxide semiconductorbody contained in said casing.
 5. A gas detecting device, in accordancewith claim 2 wherein said casing comprises two portions formed of metalwith said portions being electrically insulated from each other andserve as electrodes.
 6. A gas detecting device comprising a porous metaloxide semiconductor body including semiconductor particles whichexhibits a change in electrical conductivity when it contacts a reducinggas in air, at least one pair of electrodes attached to said body and inelectrical contact therewith, said metal oxide semiconductor body beingin the form of a layer on the surface of an insulator.
 7. A gasdetecting device comprising a porous metal oxide semiconductor bodyincluding semiconductor particles which exhibits a change in electricalconductivity when it contacts a reducing gas in air, at least one pairof electrodes attached to said body and in electrical contact therewith,and heating means for said metal oxide semiconductor body to maintain itat a predetermined temperature.